[entity] Make entity a non-static interface function as in the dune grid interface

@oliver.sander Do you think it's ok not to deprecate this? Not sure why this was static in the first place.

I suppose it was static because it could be static: All required information is in the method argument. I am not opposed to making this non-static. I am not aware of any code using the staticness of this method.

(And I don't think we need deprecation here)

I think it's good to do the same as the grid interface here and this also keeps the option open to use some internal state inside the method at some point in the future.

merged

mentioned in commit a814341c